1. Field of the Invention
The present invention relates to a dielectric ceramic and process for producing the same, in particular to a dielectric ceramic with a high relative dielectric constant, a high unloaded Q and stable temperature properties and a process capable of producing the same.
2. Description of the Prior Art
In general, it is desired to use dielectric ceramics with a high relative dielectric constant, a high unloaded Q and stable temperature characteristics for dielectric resonator systems and dielectric substrates used in signal circuits for high frequency zones such as micro wave and milimeter wave.
It is known that the unloaded Q of Ba(Zn.sub.1/3 Ta.sub.2/3)O.sub.3 dielectric ceramics varies in the wide range of from 500 to 14,000 depending on the length of heat-treating time at 1,350.degree. C. (S. Kawashima et al., "Ba(Zn.sub.1/3 Ta.sub.2/3)O.sub.3 Ceramics with Low Dielectric Loss at Microwave Frequencies", Journal of the American Ceramics Society, vol. 66, pp 421-423 (1983)). Japanese Pre-examination Patent Publication (KOKAI) No. 252008/1987 describes that the unloaded Q varies in the wide range of from 2,000 to 14,000 depending on the kind of atmosphere. Moreover, the present inventors have confirmed that not only unloaded Q but also relative dielectric constant and the temperature coefficient of resonant frequency vary depending on the time of heat treatment for firing.
As described above, since the properties of a dielectric ceramic are affected greatly by production conditions, particularly by the time of heat treatment and an atmosphere according to conventional production processes, the products having a large variation in the properties are likely to be produced. Therefore, the production process, especially the time of heat treatment and an atmosphere, needs to be controlled strictly in order to produce a dielectric ceramic with good dielectric properties. It is not easy, however, to conduct process control completely all the time, resulting in an unsatisfactory low yield of dielectric ceramics with desired dielectric properties. Accordingly, a production process has been required that is easy in process control and capable of producing dielectric ceramics having good dielectric properties in a good yield.
It is known that the difference of resulting dielectric properties according to the difference of heat treating time or an atmosphere is caused by the degree of order of crystal structure (mainly, order degree of B site ion in the structure A(B'B")O.sub.3). Heretofore, it has been thought that unloaded Q is enhanced with increase in the order; therefore, dielectric ceramics with an ordered crystal structure have been predominantly researched and developed, but dielectric ceramics with a disordered crystal structure have not drawn attention.
The U.S. Pat. No. 4,731,207 discloses a process comprising the step of heating a green compact composed of a calcined product having a composition represented by the formula: EQU xBaO.yMgO.zTa.sub.2 O.sub.5
wherein x, y and z satisfy 0.5.ltoreq.x.ltoreq.0.7, 0.1523 y.ltoreq.0.25, 0.15.ltoreq.z.ltoreq.0.25, and x+y+z=1, at a rate of from 100 to 1,600.degree. C./min. up to a temperature of from 1,500 to 1,700.degree. C., and subsequently retaining the green compact at the temperature for not less than 30 minutes. This process can produce dielectric ceramics with a fairly high unloaded Q; however, a long time is required for heat-treatment after the rapid heating step. Since the unloaded Q tends to be increased with lapse of time of heat-treatment, the ceramic with the composition of the above formula is presumably improved in unloaded Q with increase of degree of order in its crystal structure. The ceramic prepared by this process has a dielectric constant of about 25. Recently, dielectiric ceramics with a higher dielectric constant are required.